In general, an active-matrix display apparatus has a multiplicity of pixels arranged in matrix and displays an image by controlling the intensity of light pixel by pixel in accordance with image signals given. When, for example, liquid crystal is used as an electro-optic substance, the transmittance of each pixel varies in accordance with the voltage applied to the pixel. The basic operation of an active-matrix image display apparatus employing an organic electroluminescence (EL) material as an electro-optic converting substance is the same as in the case where liquid crystal is used.
A liquid crystal display panel has pixels each functioning as a shutter and displays an image by turning on/off light from a back light with such a shutter, or a pixel. An organic EL display panel is a display panel of the self-luminescence type having a light-emitting device in each pixel. Such a self-luminescence type display panel has advantages over liquid crystal display panels, including higher image visibility, no need for a back light, and higher response speed.
The organic EL display panel controls the luminance of each light-emitting device (pixel) based on the amount of current. Thus, the organic EL display panel is largely different from the liquid crystal display panel in that its luminescent devices are of the current-driven type or the current-controlled type.
Like the liquid crystal display panel, the organic EL display panel can have any one of a simple-matrix configuration and an active-matrix configuration. Though the former configuration is simple in structure, it has a difficulty in realizing a large-scale and high-definition display panel. However, it is inexpensive. The latter configuration can realize a large-scale and high-definition display panel. However, it has problems of a technical difficulty in control and of a relatively high price. Presently, organic EL display panels of the active-matrix configuration are being developed intensively. Such an active-matrix EL panel controls electric current passing through the light-emitting device provided in each pixel by means of a thin film transistor (TFT) located inside the pixel.
This active matrix type organic display panel is disclosed in Japanese Unexamined Patent Publication No. 8-234683. An equivalent circuit for one pixel of the display panel is shown in FIG. 62. A pixel 16 is provided with an EL element 15 serving as a light emitting element, a first transistor 11a, a second transistor 11b, and capacitance 19. The light emitting element 15 is an organic electroluminescence (EL) element. In this invention, the transistor 11a which supplies (controls) a current to the EL element 15 is referred to as a driving transistor 11. Also, a transistor which operates as a switch, such as the transistor 11b of FIG. 62, is referred to as a switching transistor 11.
Since the organic EL element 15, in general, has rectification property, it is called OLED (organic light emitting diode) in some cases. In FIG. 62, the EL element is denoted by OLED 15 of which D indicates diode.
Note that the light emitting element 15 of this invention is not limited to the OLED, and other light emitting diodes are usable so far as a luminance thereof is controlled by adjusting an amount of a current supplied thereto. For example, an inorganic EL element is also usable. Another example may be a white light emitting diode made from a semiconductor. Yet another example may be general light emitting diodes. A light emitting transistor may also be usable. The light emitting diode 15 does not necessarily show the rectification property. A bidirectional diode may be used as the light emitting diode 15.
In the example shown in FIG. 62, the source terminal (S) of p-channel transistor 11a is connected to Vdd (power source potential), while the cathode (negative electrode) of the EL device 15 connected to ground potential (Vk). On the other hand, the anode (positive electrode) is connected to the drain terminal (D) of the transistor 11b. The gate terminal of the p-channel transistor 11b is connected to a gate signal line 17a, the source terminal connected to a source signal line 18, and the drain terminal connected to the storage capacitor 19 and the gate terminal (G) of the transistor 21a. 
In order to operate the pixel 16, first, the source signal line 18 is applied with an image signal indicative of luminance information with the gate signal line 17a turned into a selected state. Then, the transistor 11b becomes conducting and the storage capacitor 19 is charged or discharged, so that the gate potential of the transistor 11a becomes equal to the potential of the image signal. When the gate signal line 17a is turned into an unselected state, the transistor 11a is turned off, so that the transistor 11a is electrically disconnected from the source signal line 18. However, the gate potential of the transistor 11a is stably maintained by means of the storage capacitor 19. The current passing through the EL device 15 via the transistor 11a comes to assume a value corresponding to voltage Vgs across the gate and the source terminals of the transistor 11a, with the result that the EL device 15 keeps on emitting light at a luminance corresponding to the amount of current fed thereto through the transistor 11a. 
As described above, according to the prior art configuration shown in FIG. 62, one pixel comprises one selecting transistor (switching device) and one driving transistor. Another prior art configuration is disclosed in Japanese Patent Laid-Open Publication No. HEI 11-327637 for example. This publication describes an embodiment in which a pixel comprises a current mirror circuit.
With the method shown in FIG. 62 of outputting an image signal as a voltage from a source driver 14, an output stage impedance of the source driver 14 is low. Therefore, it is easy to program the image signal to the source signal line 18.
With the method of outputting an image signal as a current, such as a current mirror structure shown in FIG. 1 or disclosed in Japanese Patent Application No. 11-327637, an output stage impedance of a source driver 14 is high. Therefore, it is undesirably difficult to program the image signal to the source signal line 18 in a black display region. FIG. 2 is an illustration of a reason for the difficulty.
In order to cause the light emitting element 15 of each of the pixels 16 of FIG. 2 to display, the transistors 11b and 11c are brought into the conductive state by the gate signal line 17a in one horizontal scan period, so that a current Iw is drawn from the power source Vdd to the source driver 14 via the driving transistor 11a and the source signal line 18. Gradation display is performed in accordance with an amount of the current drawn to the source driver 14. A charge responsive to the gate voltage corresponding to the drain current of the transistor 11a is accumulated in the capacitance 19.
Then, the transistor 11d is brought into the conductive state by the gate signal line 17b, and the transistors 11b and 11c are brought into the non-conductive state by the gate signal line 17a, whereby a current responsive to the charge in the capacitance 19 flows from the Vdd to the light emitting element 15 via the transistor 11a. 
The current flowing to the source signal line 18 changes gradually depending on a product of stray capacity (stray capacity) 641 of the source signal line 18 and source-drain (S-D) resistance of the transistor 12. Therefore, when the capacitance 641 and the resistance are increased too much, the current sometimes fails to reach a predetermined value in one horizontal scan period.
With a reduction in the current flowing to the source signal line 18 (in the case of low gray scale), the source-drain resistance of the transistor 11a is increased; therefore, time required for the current to change is increased with the reduction in the current. Though it depends on diode characteristics of the transistor 11a and a value of the stray capacity 641, it takes 50μ seconds for the current flowing to the source signal line 18 to change to 1 μA, and it takes 250μ seconds for the current to change to 10 nA, for example.
The current flowing to the source signal line 18 supplies a charge to the source signal line 18 from the Vdd via the transistor 12a to change the charge of the stray capacity 641, so that a voltage of the source signal line 18 is changed to change the current flowing through the transistor 12a (the current flowing to the source signal line 18). Since a quantity of the supplied charge is small in a region where the current is small, the voltage change on the source signal line 18 is slowed down to delay the change in the current.
Thus, it has been impossible to reduce the horizontal scan period, and, depending on the number of display columns, flickering occurs due to the reduction in frame frequency.